Amplifier



April 29, 19411 A v. M. couslNs 2,240,286

A AMPLIFIER I Filed Feb. 2, 1940 I l l V Y V l5 l Il ff) ff/ /f2 [All VVVV

MAI

/NVENTOR L/A l. COUS/NS A 7' TOR/VE V Patented Apr. 29, 1941 AMPLIFIER Van M. Cousins, Lyndhurst, N. J., assigner to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New vYork Application February 2, 1940, Serial No. 316,926

6 Claims.

This invention relates to amplifiers and particularly to amplifiers using stabilizing negative feedback.

It is the usual practice to filter the power supply to the low level stages of an amplifier to a very high degree to prevent the introduction of I ventional type of amplifier, it is frequently economical and desirable for'circuit reasons to connect the feedback circuit to the primary side of the output transformer, that is, tothe plate electrode of the output tube. While this type of feedback circuit largely eliminates the phase shift of the transformer lfrom the feedback loop and, therefore, increases the margin of the circuit against instability', it vhas the disadvantage of producing a higher level of power supply noise in the output than When^no feedback is used. The reason for this increasev in noise is fully explained in a copending application of J. B. Harley, Serial No. 316,913, led February 2, 1940, but, briey stated, it is due to the fact that the noise potential drop in the output tube is fed back and amplified vand produces across the output transformerV a noise potential which is' in phasev with Vthe noise potentials across the transformer due directly to' the imperfectly filtered space currentilowing through it.

It is the object of this invention to eliminate this excess noise in certain types of feedback amplifiers. Y

In accordance with the general features of the invention, the feedback amplifiers of the general type discussed above are provided with a second feedback .path which impresses on the input circuit other Ynoise potentials which oppose the effect produced by those in the signal feedback path and .thereby reduce the noise to,'or even below, the :level obtaining-.in the amplifier without feed back. f

In the case of a push-pull amplifier with conventional feedback connection from the plate of each output tube to a push-pull 'circuit at a lower signal level, such, for example. as an input stage, the noise feedback may take the form of a mid-branch connection to a suitable impedance common to the input circuit of the input tubes.

'When a single input tube is used to drive pushpull output tubes through an inverter stage, the two signal feedback paths may be connected from theplates of the push-pull tubes to the input tube in such a way that both paths are effective as signal feedbacks and also effective in combination to limit the noise level to that obtained without any feedback.

In .the drawing, Y

Fig. l is a push-pull feedback amplifier with means for reducing power supply noise according to the invention; and A Fig.y 2 is an inverter tube amplifier with two signal feedback circuitswhich oppose each other with respect to power supply noise.

The amplifier vof' Fig. l comprises an `input transformer I I, two push-pull stages of amplification, an output transformer I 2 and the power supply unit I3 adapted to be energized in the usual manner from an alternating current source I 4. The input tubes I5 and I6 are resistancecapacity coupled to thev output tubes I1 and I8 andall four tubes are self-biased in the usual manner by means of resistors in the cathode circuits. The plate current supplied to the output' tulbes over thev conductor I 9 is filtered by the choke coil 20 and condenser 2I which reduces the ripple potential from the rectifier I3 to a value which i's tolerable inthis high level stage. The usual additional filtering is 'provided' by resistors 22, 23, 24, 32 and 33 and condensers 34, 35 and 25 for 'the'plate and screen currents of the input tubes`I5 and I6 andthe screens of the output tubes II and I8. f

A stabilizing negativefeedback path of the typeshown in Patent 2,123,241 to Harley, July l2, 1938, extends from the plate of the tube I1 through a. suitable resistor- 26 to the cathode of tube I5, whereby signal currents in the output tube produce across the resistor-21 in the inputY of tube.v I5 a ,potential-varying with the output toV reduce the amplifier gain linthe .well-known manner. A `similar path is providedfrom the plate-of tube I8 through resistor 28 to develop a feedback 'potential across the resistor 29. Any ripplevvoltage across the condenser 2| in the output of the power supply filter will cause ripple currents to flow through the primary windings of the output transformer I2 and the tubes Il and VIll to ground. Since thefeedback paths are connected to `the plates of tubes I'I and I8, the ripple potential drops across the tubes are fed tube 4|, push-pull output tubes 42 and 43 and an inverter tube 44 coupling the tube 4| to the tube 43 in the well-known manner. The amplifier is energized from a conventional power supply unit comprising a transformer 45 adapted to be connected to an alternating current line, a rectifier 46 and suitable filter elements for reducing the amplitude of the ripple components in the rectifier output. The plate circuits of the output tubes are energized over conductor Y,4|. which is connected to the choke coil 4B but is well known that a shunt feedback of the typeunder discussion here reduces the eective plate impedance of the tube from which the feedback l takes place.

and since the feedback effectivelyreduces the As stated above, a certainripple current flows through the transformer'primariesv and tubes and I8, in the absence of feedback the value of the ripple `current in thel transi.

former. In other words, the ripple components, due to the feedback action, must be in phase with the ripple current already flowing in the winding.

These noise currents will, of course, beof opposite phases in the two sections of the primary winding of the transformer, due to the push-pull connection, and will, therefore, tend to cancel each other in their effects in the level of the noise in the output circuit. In practice, however, there is never a perfect balance in the transformer itself or in the amplifier stages and, since the ripple voltages pass through both stages in passing around the feedback loop, all of the unbalances of the amplifier may tend to increase the effective unbalance at the output transformer and thereby increase the noise in the associated load circuit.

In this circuit, however, the mid-point ofthe primary winding of the output transformer is' connected through resistor 30 to the grid end of resistor 3| in the common portion of the input circuits of the tubes |5 and I6, therebyV condenser 2| and of a polarity such as to opposeY the ripple potentials across the resistors 21 and 29 in the input circuits of these tubes. By p-roper choice of the values of resistors 30 and 3|, the ripple potential developed across 'resistor 3| may be made just sufficient to balance the ripple potentials across resistors 21 and 29 `so that no increase in output noise results from the use of negative feedback. It may even be possible in some cases to reduce the noise level further by developing across resistor 3| a slightly greater potential which will also neutralize the ripple voltage drop which exists in the output transformer in the absence of feedback.V This mid-branch connection also serves another very useful purpose. The condenser 2| in the filter network is ordinarily of large capacity and hence of very low impedance for signal frequencies. At extremely low frequencies, below the band of interest, however, potentials of considerable magnitude may beY developed across this condenser and impressed on the input circuit through the signal feedback paths in such phase as to produce low frequency oscillations.

In this circuit these low frequency potentials re also fed back over the mid-branch connection and impressed on resistor 3|, thereby introducing a compensating eect which stabilizes the circuit against troubles of this type.

The circuit of Fig. 2 comprises a single input the current for the plate circuits of the tubes 4|y and 44 is subjected to further filtering, in

accordance with the usual practice, by the resistors 4 9, 56 and the condenser 5|. The screen Vcurrents are similarly filtered by resistor and condenser 66.

As pointed out above, when the stabilizing negativefeedback is applied to an amplifier by means of the feedback connection from the plate of the output tube, it ordinarily results in an increase to the noise level and requires additional circuit connections for balancingout this increase in noise. Applicant, however, uses feedback connections of this type inthe circuit of Fig. 2 in sucha way that the noise is kept at at low level without any additional circuit features. One feedback path comprising resistor 52 andcondenser 53 connects the plate of tube 42 to' the cathodeof tube 4| so that a voltage proportional to the output of tube 42 is developed across the resistor 54 and a second feedback path, comprising resistor 55 and condenser 56, connects the plate of tube v43 to a second grid, such as the screen grid 51 of the tube 4|, and extends back to the cathode 58 of tube 43 through resistor 6|, condenser 66 in the power supply filter and the by-pass condenser 12 in the output stage. The potential variations developed across the resistor 6| are effective to vary the potential of the screen grid 51 but the screen grid 60 of the inverter tube 44 is maintained substantially at ground potential for signal currents because of the'low` impedance of condenser 66, The resistors 52, 54 and 55, 6| are so proportioned with respect to the characteristics of the tube 4| and its associated circuits that the two feedback paths produce substantially equal reductions' in the gain of the amplifier. While condensers 53A and 56 have been shown in the feedback paths, they are not essential and, if desired, they may be omitted if the necessary changes are made in the values of the resistors 52, 55, 54 and 6|. If used,` they may be preferably of such capacity as to have appreciable impedance only for frequencies up to about cycles per second so as to reduce the feedback at these frequencies Vand thereby improve the low frequency response of the amplifier. The primary of the output transformer 62 is shunted by a network, comprising two condensers 63, 63 and a resistor 64, which protects the output tubes from damage and reduces the noise due to the transient currents which may be set up when either output tube is biased to cut-ofi by a signal of abnormally high level. In order to be effective for these purposes, the condensers 63, 63 are usually low venough in impedance to have some undesirable shunting effect on the higher signal frecuencia-is. This high frequencyv attenuation may be compensated conveniently by making condenser 59 of sufficient capacity to reduce the feedback applied to the screen grid 51,'by a corresponding amount in this part of the frequency range,

Since the signal feedback potential in the two paths are of opposite phases by virtue of the connections to the plates of the push-pull output tubes and are applied to the cathode and screen grids, respectively, of tube 4|, they will both operate to reduce the gain of the circuit. The ripple current in these paths due to imperfect fltering of the power supply, will, however, be of the same phase and will, therefore, produce equal and opposite effects on the plate current of the tube 4| and thereby prevent any increase in the noise level.

It is Well known that the phase shift around the feedback loop of a negative feedback amplier increases with frequency and may finally become positive at some frequency which is preferably above the range of interest. When an amplifier, such as the circuit of Fig. 2, is designed to transmit a wide frequency band, this positive feedback may be sufficient to produce instability at frequencies of the order of 50 kilocycles. It has been found that this type of instability. is effectively prevented in this circuit by connecting a condenser 1l directly between the plates of the input and inverter tubes. In this particular circuit a condenser of 50 micro-microfarads greatly improves the stability of the circuit without introducing appreciable loss at audio frequencies.

While the invention has been described for purposes of illustraiton with reference to particular circuits, it will be understood that lthese circuits may be modied in various ways. The general principle of using a plurality of negative feedback circuits which oppose each other with respect to power supply noise is, of course, not limited in its application to circuits of the pushpull type but may be applied to amplifiers in general.

In the circuit of Fig. 2 the feedback to the screen grid 5l of 4the -tube 4l may instead be apl plied, if desired, to the control grid 'l0 by inserting a suitable resistor between the lower end 61 of the input shunting resistor 68 and ground and connecting the feedback circuit to the point 61. While this modification would be quite satisfactory if the amplifier is always used with an input transformer, as shown, it would not be desirable for an amplifier which may be resistance-capacity coupled in the ordinary manner to a driver stage, since in that case the impedance of the driver tube would be effectively in shunt to the added feedback resistor and any changes in the tube impedance would destroy the balance between the two feedback paths.

Other possible modifications of these circuits will occur to those skilled in the art but the invention is intended to be limited only by the scope of the following claims.

What is claimed is:

1. In an amplifier, two vacuum tubes having input and output circuits connected in push-pull relation, a vacuum tube input stage having a grid circuit connected to a source of signals and a plate circuit connected to the push-pull tubes, a source of direct current for the amplifier having undesirable noise components, an impedance in the push-pull output circuit, a connection from the direct current source to the mid-point of the impedance, a negative feedback circuit from at least one point on the impedance to the input stage whereby the noise components are fed back along with the signals and amplified and assume in the output impedance a larger value than they would have in the absence of feedback and a path from another point on `the impedance to the input stage for impressing thereon noise components for neutralizing the effect produced by the noise components in the feedback path.

2. In an amplifier, input and output stages each having two vacuum tubes connected in push-pull relation, a source of direct current for the amplifier having noise components, plate electrodes in the output tubes, an output impedance connected between the electrodes with its midpoint connected to the source of current, negative feedback sign-al paths extending from the electrodes to the input circuits of the input tubes and impressing thereon along with the signal noise components from the source which are normally amplified and increase the noise level in the output of the amplifier and a feedback connection from an intermediate point on the output impedance to both of the input circuits impressing thereon noise components opposing the noise components in the feedback paths in their effects on the noise level of the amplifier.

3. In an amplifier, an input tube, push-pull output tubes, means including an inverter tube coupling the input tube to the output tubes, plate electrodes in the output tubes, an outputimpedance connecting the electrodes, a source of direct current having undesired noise components, a connection between the source and the mid-point of the impedance and two feedback paths from the plate electrodes to the input tube carrying signal energy of different phases and noise components of substantially the same phase and means for utilizing the signal energy in both paths to reduce the gain of the amplifier and for causing the noise components of one path to oppose the effect on the input tube produced by the noise components in the other path.I

4. An amplifier according to claim 3 in which the input tube has a signal grid and at least one other grid and one of the feedback paths is connected to the other grid.

5. An amplifier according to claim 1 having -a resistance-capacity network shunting the output impedance for protecting the output tubes from excessive transient voltages and a capacity shunting one of the feedback paths and selectively reducing the amount of feedback at high frequencies to compensate for the loss at high frequencies produced by the resistance-capacity network.

6. An amplifier according to claim 3 `in which the input tube and the inverter tube have plates and cathodes connected in push-pull relation with a condenser of very small capacity connected directly between the plates and stabilizing the amplifier against singing at super-audible frequencies.

VAN M. COUSINS. 

